Aurelia aurita
Spot Common Moon Jellyfish drifting through Zeeland, British Columbia, and Croatian waters. Most sightings fall between 6-18°C. Here's where and when to find them.
Safety warning
This species can sting on contact with its tentacles, which may trail well beyond the body and can still sting after the animal is dead or washed up. Look but do not touch, and give it a wide berth.
If you're stung
Stinging tentaclesGeneral first-aid guidance, not a substitute for professional medical care. In an emergency, call your local emergency number.
A saucer-shaped bell up to about 35 cm across, translucent white to pale blue, with four clearly visible pinkish or purple horseshoe-shaped gonads (reproductive organs) showing through the top of the bell, that four-leaf-clover pattern is the single best identification mark. The tentacles are short and fringe-like rather than long and trailing. In cooler, dimly lit water the bell can look almost colourless, so look for the gonads rather than the tint.
Moon jellyfish drift passively with currents rather than swimming with any real purpose, so encounters feel unhurried, they simply float past you. They feed by trapping tiny plankton on a layer of mucus covering the bell and sweeping it towards the mouth with their short oral arms; watch for that slow, rhythmic pulsing as you approach. They take no notice of divers whatsoever, which makes them easy to observe closely without any change in their behaviour.
During population blooms, aggregations can number in the millions and are visible from the surface as a pulsing carpet, these blooms are partly triggered by nutrient-rich, warmer coastal water and have been recorded clogging industrial water intakes, a reminder of how ecologically influential a single species can be.
Northern Hemisphere sightings are spread broadly from April through to February, suggesting presence across most of the year in cooler temperate regions. Bloom events, when numbers surge dramatically, tend to cluster in warmer months of the local season, so late spring through summer is the most reliable window in European and North American waters.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most commonly logged in the Netherlands (particularly Zeeland), the United Kingdom, and along the British Columbia coast in Canada, with a strong presence right across coastal Croatia and France. It turns up throughout temperate North Atlantic and North Pacific waters, so divers exploring Scottish sea lochs, the Pacific Coast of the United States, or Central Dalmatia's islands are all in productive territory. Sightings are recorded right into estuaries and sheltered bays, so there is no need to head offshore.
Mapping where Common Moon Jellyfish is logged…
They are most reliably logged in Zeeland in the Netherlands, along the British Columbia coast in Canada, and throughout coastal Croatia, particularly Central Dalmatia. Strong sightings also come from Scotland, Brittany and Normandy in France, and the Pacific Coast of the United States. They favour cool, sheltered coastal and estuarine waters.
In the Northern Hemisphere they are present across a long season stretching from April well into winter, but the biggest aggregations typically appear in late spring and summer when nutrients in coastal water peak and trigger blooms. If you want numbers rather than the occasional drifter, aim for May to August in European and North American destinations.
Look for the four pink or purple horseshoe shapes visible through the top of the translucent bell, no other common jellyfish in temperate waters shares that four-leaf-clover pattern. The bell is broad and flat, the tentacles are short and fringe-like, and the whole animal rarely exceeds about 35 cm. If the horseshoes are there, you have your ID.
No. Their sting is so mild that most divers with normal skin feel nothing at all. They make no effort to approach you and will simply drift past. There is no special behaviour needed around them, though bare-skinned contact on very sensitive areas like lips or eyes can occasionally cause mild irritation.
A typical adult bell measures around 25 to 35 cm across, roughly the size of a dinner plate. Some individuals reach 40 cm in particularly productive waters, but anything larger than that is unusual.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
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